(1) Field of the Invention
This invention relates to, for example, automatic frequency control circuit and method of automatic frequency control preferable for use with portable telephone terminal.
(2) Description of the Related Art
Generally, automatic frequency control circuit (AFC circuit) is a circuit which controls reference frequency in the synthesizer (received local generation unit) of terminal station (portable station) according to the quantity of a drift, after counting stable terminating frequency from the base station, when a drift with predetermined theoretical value exists. Also, terminating frequency is configured to be counted at the point of received intermediate frequency signal (hereinafter called as received IF signal).
Because the status of radio wave at the portable station (portable telephone terminal) is always changing by receiving fading phenomena, and etc., to average these effects, sufficient counting time is necessary for precise measurement. On the other hand, the equipment, like a digital portable telephone, which operates with TDMA (Time Division Multiple Access) system during receiving period, can only measure frequency of receiving slot of itself during TDMA operation (that is, during conversation), thus, longer counting time is also necessary for sufficient counting process. Therefore, a longer time (T/S).times.F becomes necessary, where T is a time, F is a time of one frame, and S is a time of the receiving slot of itself.
Because of this, in portable station, there are many cases counting not received IF signal itself, but reference carrier of detected signal which is regenerated in the demodulator circuit (DEM circuit) in receiving unit of portable station. That is to say, because influence of fading is suppressed, regenerative carrier signal can be counted more precisely within short period of time than counting received IF signal directly.
However, as stated above, in case of automatic frequency control circuit counting reference carrier of detected signal regenerated in demodulator circuit, the limit of frequency follow up of the demodulator circuit becomes the limit of frequency follow up of the automatic frequency control circuit itself. That is to say, the demodulator circuit, in most cases, is simplified as not to be equipped with unnecessary circuit other than enough circuit for detection, in order to reduce the scale of the circuitry, and to easily integrate in large scale. Therefore, frequency limit of follow up as stated above has been produced. In certain instances, there may be some cases when the frequency limit of follow up will theoretically be produced by the detection system.
Specifically, as shown in FIG. 9, the carrier regeneration circuit which is configured with the digital circuit of demodulator circuit has the limit of follow up range ["-FL.about.+FL" (2.vertline.FL.vertline.) cf. A in FIG. 9] against input frequency (received IF frequency). With the input frequency which is out of this limit of follow up range, output frequency (regenerative carrier frequency) repeats identical value. Therefore, in the area out of the limit of follow up range, ratio of value between output frequency and input frequency does not coincide with. On the other hand, when a synthesizer using highly stable local oscillator is equipped in portable station, this synthesizer becomes expensive, and the total cost of whole portable station will be boosted in turn.